1. Field of the Invention
The invention relates to the field of sensors designed to measure the speeds of turboengine shafts and turbojet shafts.
2. Description of the Prior Art
At the present time, the speed of turboengine or turbojet shafts is checked by standard electromechanical devices that are known as "phonic wheels" which can be likened to homopolar generators.
The drawing of a device such as this is shown in FIG. 1.
These homopolar generators are formed by (see FIG. 1):
a rotating part formed by a magnetic steel wheel with Z teeth, rotationally linked to the shaft to be checked (not shown); PA1 fixed part constituted by a coil with magnetic core 2, to which there is coaxially bonded a magnet 3. PA1 it is difficult to make measurements at low speed: the analog signal is then a low amplitude signal and it can be below the level of the ambient electromagnetic noise; PA1 the sensor has high sensitivity to induced or conducted electromagnetic parasites; PA1 it has low resistance to lightning.
The assembly is positioned in such a way that the passage of the teeth creates flux variations in the gap 4, the result of which is a voltage induced at the terminals of the coil.
The frequency of the signal 5 collected is equal to: ##EQU1## where N is the speed of rotation of the shaft in rpm. Its amplitude is an increasing function of the speed of rotation.
The sturdiness of this type of sensor enables it to stand up to the severe environmental constraints (vibrations, temperature and pollution) to which turbo-machines are subjected. However, such sensors have certain limitations, for example:
The sensor according to the invention is aimed at overcoming these limitations while at the same time preserving the qualities of the electromagnetic sensor.
Another aim of the invention is to provide these advantages in such a way that existing sensors can easily be replaced.